1. Field of the Invention
The present invention relates especially to a method and a system for the protection of compressed data against errors.
It can be applied in the field of lossy transmission caused by the transmission medium (for example in wireless transmission).
It can be applied for example lossless or lossy image transmission systems using the JPEG 2000 standard.
The invention can also be used in any data transmission system whose format comprises a header including markers related to the data that follow the header.
2. Description of the Prior Art
In the JPEG 2000 (ISO/IEC 1544) standard, there are tools for taking account of errors that may result from the transmission of bitstreams or codestreams or files in an environment propitious to errors. These tools described in the T.800|ISO/IEC 154444-1 recommendation are used to detect the presence of errors during the entropic decoding of a JPEG 2000 decoder and/or to resynchronize the decoding method in the case of losses of data packets. These tools, however, cannot be used to prevent the presence of errors in the headers (main header or tile header) or correct errors that may be present in the codestream.
Modifications in the method of entropic encoding/decoding can be used either to augment the mechanism of taking account of the errors proposed in the JPEG 2000 standard and/or obtain an error protection mechanism. The advantage of these techniques is that it provides a very efficient compromise between the added redundancy and protection and error decoding capacity. The resulting codestream unfortunately is then not compliant with the definition of a JPEG 2000 (Recommendation T.803|ISO/IEC 154444-4) mechanism for taking account of errors.
JPEG 2000 also includes many options used to build and order compressed data in different ways. In particular, it assembles information.
After a particular ordering and formatting of the data structure, it is possible to separate the JPEG 2000 codestream into several parts. Each of these parts has a different degree of sensitivity to errors. Thus identified, each part can then be protected according to its level of sensitivity and its importance in the stream in using different redundancy levels. This enables the setting up of a data protection mechanism that is unequal for the different parts of the stream, thus greatly improving performance in terms of bandwidth or power consumed relative to the transmitted payload information. The drawback of the use of such a method is that it generally relies on the separation of the streams of different sensitivity, which are encoded and transmitted separately. Since different parts of the data stream cannot be decoded separately, it then proves to be necessary, after the error correction decoding, to again combine the data to regenerate a JPEG 2000 stream (compliant with the T.803|ISO/IEC 154444-4 recommendation).
The reference Poulliat.C, Vila P., Pirez D., Fijalkow I., entitled “Progressive JPEG 2000 Image Transmission over noisy channel”, Eusipco 2000, Toulouse, France, 3rd-6th Sep. 2000, describes a solution of this kind for the unequal protection of data using RCPC (Rate Compatible Punctured Convolutional) codes.
The document by Ambarish Natu, David Taubma, “Unequal Protection of JPEG 2000 Code-Streams in Wireless Channels”, IEEE GLOBECOM'2, discloses another solution of unequal protection against errors. The method uses Reed Solomon codes to protect a JPEG 2000 codestream against errors. In this document, the protection of the header against possible errors is not evoked, and the redundant information is used solely by data packets of the codestream. The packets are modified to include the redundant information. The resultant codestream is not compliant with the mechanism for taking account of errors defined in the T.803|ISO/IEC 15444-4 recommendation.
The object of the present invention relies especially on a novel approach for which the redundant information used for error correction and/or decoding is inserted into the header.